1. Field of the Invention
The present invention relates to an LCD device and an LCD control method, particularly to an LCD device free of retained images and a control method thereof.
2. Description of the Related Art
Because of slimness, lightweight, no radiation, low power consumption, long service life, soft images and ocular health, LCD (Liquid Crystal Display) has high market share in many application fields. Among LCDs, the active-matrix TFT LCD is the mainstream of the market.
Refer to FIG. 1 for a conventional TFT LCD driver circuit. In a LCD panel 10, there is a plurality of arrayed pixels 11, and each pixel 11 has a TFT (Thin Film Transistor) 12 functioning as a switch. The gate of the TFT 11 is connected to a horizontal scan line 13, and the drain is connected to a vertical data line 14, and the source is connected to the electrodes (such as the pixel electrode) of a liquid-crystal capacitor 8 and a storage capacitor 9, wherein the other electrodes (such as the common electrode) of the liquid-crystal capacitor 8 and the storage capacitor 9 are connected to a common voltage Vcom (the Cs-on-common type). In another type of LCD panel (not shown in the drawings), the other electrode of the liquid-crystal capacitor 8 is connected to a common voltage Vcom, and the other electrode of the storage capacitor is connected to a horizontal scan line 13 (the Cs-on-gate type). The data lines 14 are driven by a source driver 15, and the scan lines 13 are driven by a gate driver 16. Refer to FIG. 2. An LCD display device 23 comprises an LCD panel 10 and a control system driving the LCD panel 10. In the LCD device 23, an input interface 17 provides power source for a DC (Direct Current) power converter 18, and then the DC power converter 18 provides working voltages for a gamma voltage generator 19, a clock controller (Tcon IC) 21 and a common voltage circuit 20. The gamma voltage generator 19 outputs a reference voltage to a source driver 15. The source driver 15 generates a gray-level voltage to the pixel electrode of a pixel in the LCD panel 10 according to the reference voltage and a pixel data signal from the clock controller 21. The common voltage circuit 20 generates a common voltage Vcom to the common electrode of the pixel. Thus, a voltage drop is created between two sides of the liquid crystal capacitor to twist the liquid crystal molecules and create an image. The clock controller 21 controls the operations of the gate driver 16 and the source driver 15 according to the pixel data signal and an external clock signal provided by the input interface 17. Thereby, the control system can control the pixels 11 of the LCD panel 10 to operate according to data signals and present images on the LCD panel 10.
In detail, the gate of TFT LCD turns on or turns off to charge or discharge the pixel electrode of the crystal liquid capacitor. When a voltage high enough (such as Vgh) is applied to a scan line, all the gates on the scan line are turned on, and the related data lines respectively write the corresponding pixel gray-level voltages into the pixel electrodes at the same time. After an appropriate charging time, a voltage small enough (such as Vgl) is applied to the scan line to turn off all the gates on the scan line, and the charges of the pixel electrodes are kept for a period of time. Such a design is the so-called holding type display. In normal operation, the gate is turned on and off persistently, and the pixel data is also renewed constantly. When the LCD display is switched off (or shut off) by user, all the voltages of the control system become zero. Thus, all the gates on the scan lines no more turn on, and the pixel electrodes keep the voltages of the last image until the charges are completely released by natural current leakage. As the charges of the pixel electrodes are released slowly and unevenly, retained images appear on the screen of the LCD panel. Thus, the screen turns from black to pale, and the retained images appear in different positions and have different areas and diverse chroma. The retained images phenomenon depends on the charge keeping capability of the pixel.
At present, a reset IC 22 is arranged in between the input interface 17 and the gate driver 16 to solve problem of retained images. In the instant of switching off LCD, the reset IC 22 detects the voltage variation of Vdd. The Vdd variation will trigger the reset IC 22 to set the XAO signal to a low level. According to the XAO signal, the gate driver 16 makes all the scan lines output a Vgh voltage to turn on all the gates of TFT at the same time. Thus, the charges of the pixel electrodes of all the pixels on the LCD panel 10 are released via the data lines to accelerate the disappearance of retained images.
In another solution, a clock controller (Tcon IC) is used to detect the turning off of the LCD display device, and then black images are sequentially insert into the rows of pixels; after the liquid crystal of the liquid crystal capacitors of all the pixels on the panel has been twisted to an identical angle, the charges are released. Thus is also solved the problem of retained images.
The abovementioned reset IC can indeed solve the problem of retained images. However, it increases the material cost. Besides, the inrush current will be very high in turning off the system because all the scan lines output voltage Vgh at the same time. The inrush current makes electric punctures likely to occur between the Vgh pads and the conductive bumps of COG IC. Further, turning off and turning on the LCD device within a short interval of time results in a power-dip problem. Moreover, the method of sequentially writing black images into pixel rows also has problems of slow retained image elimination and uneven discharging rates of pixels.
Accordingly, the present invention proposes an LCD device and a control method thereof to effectively eliminate retained images.